Charge injection and transport in tetra-phenyl-porphyrin C. Calcavento a , G. Conte a,* , S. Salvatori a , R. Paolesse b , M. Berliocchi c , A. Di Carlo c , P. Lugli c , A. Sassella d a INFM and Department of Electronic Engineering, University of Rome ‘‘Roma Tre’’, Via della Vasca Navale, 84-00146 Rome, Italy b Department of Chemical Science and Technology, University of Rome ‘‘Tor Vergata’’, Via della Ricerca Scientifica, 00133 Rome, Italy c INFM and Department of Electronic Engineering, University of Rome ‘‘Tor Vergata’’, Via del Politecnico, 1-00133 Rome, Italy d INFM and Department of Material Science, University of Milan ‘‘Bicocca’’, Via R. Cozzi, 53-20125 Milan, Italy Abstract The analysis of charge injection and transport in 5,10,15,20-tetra-phenyl-phorphyrin (H 2 -TPP) has been analysed by I(V , T) measurements in the 50–350 K range. Different metals have been used to study the influence of the metal/organic interface on the transport properties. Organic films have been deposited by spray coating and organic molecular beam deposition (OMBD). The observed results show ohmic transport at each temperature for thin films deposited by both the techniques. Two injection mechanisms have been evidenced: thermionic emission over a barrier independent of the metal contact for T > 250 K and tunnel effect at the lower temperatures. On the other hand, a different behaviour has been evidenced on thick films deposited by spray coating. Here, the dark current as a function of the applied voltage increases linearly at the low electric fields independent of the metal contact and shows the same temperature dependence observed for thinner films. At high voltage a non-linear field dependent transport mechanism has been seen. Moreover, the tunnelling contribution to the current increases with the metal work function. The observed trends have been analysed on the basis of injection limited transport in organic amorphous semiconductors. The resistance of interface states and deposition induced defects are considered to discuss the applicability of the Anderson principle for band alignment and justify the observed results. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Charge injection; Transport; Interface properties; Metal/organic junctions; Thin films 1. Introduction The importance in photosynthetic processes and biologi- cal systems have made porphyrins the subject of many chemical-physics studies. Moreover, porphyrins are promis- ing materials in a large number of applications, like che- mical sensing [1] and molecular electronics [2]. The synthesis of these compounds is a well-known technique and substitutes can be easily introduced in the base mole- cule, as well as metals into the central ring, in order to change the optical and electronic properties [3]. 5,10,15,20- Tetra-phenyl-porphyrin (H 2 -TPP) shows considerable pos- sibility for the development of electronic devices owing to the large number of p-conjugated bonds in the molecule. On the other hand, H 2 -TPP is considered a large bandgap organic semiconductor with a p-type like transport. Similar to inorganic materials, the realization of metal contacts with p-type molecular semiconductors is an hard task, generally leading to Schottky junctions formation influenced by the presence of interface states. Crucial to the development of electronic devices is the knowledge of the charge transport mechanisms inside the material and the junction properties of the organic/metal barrier formed. The properties of Schottky junctions formed by vacuum-sublimed H 2 -TPP films with different metals have been analysed by Harima et al. [4] by using photocurrent spectroscopy measurements. These authors have shown that the band alignment is not influenced by surface states as generally observed for inor- ganic semiconductor-metal barriers. The energy structure of vacuum-sublimed H 2 -TPP films has been analysed by Ishii and Seki [5] by using UPS measurements. A possible role of interface states in determining the Fermi level alignment and the applicability of the Bardeen theory [6] for Schottky barrier formation has been evidenced by these authors. The energy levels structure of the H 2 -TPP molecule and the organic/metal barrier has also been inferred by the diffusion potential analysis and by the Kelvin probe technique by Harima et al. [7]. These authors have shown that the Fermi level position for the intrinsic material is localised 0.6 eV over the H 2 -TPP valence band, assuming the edge at 5.4 eV below the vacuum level as evaluated by UPS measurements. Synthetic Metals 138 (2003) 255–260 * Corresponding author. Tel.: þ39-6-5517-7268; fax: þ39-6-5579-078. E-mail address: gconte@ele.uniroma3.it (G. Conte). 0379-6779/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(02)01307-3